This picture of two mating insects comes with an unexpected twist. The one on top is a female, and she has a penis. The one on the bottom is a male, and he has the equivalent of a vagina. During sexual bouts that can last for 40 to 70 hours, she penetrates him and uses her genitals not to deliver sperm, but to collect it.

The 3-millimetre-long species is called Neotrogla curvata, and it lives in Brazilian caves. There are four Neotrogla species and the females all have penises. They belong to a group of insects called barkflies or barklice—closely related to lice and true bugs, and only distantly related to actual flies.

Caver Rodrigo Ferreira discovered these insects several years ago, and sent them to Charles Lienhard, a barklice specialist, for further study. He gave them their name, and also noted their unusual reversed genitals. Working with Lienhard and Ferreira, Kazunori Yoshizawa has now shown that the female’s sex organ doesn’t just look like a penis—it acts like one too.

Neotrogla’s set-up is very different. The female has a penis-like protrusion called a gynosome, which is erectile and curved. The male has no such organ; he has an internal chamber instead. When she penetrates him during sex, he delivers sperm into a duct in her gynosome, which leads to a storage organ. He still ejaculates, but he does so inside his own body, not hers.

Cross-section showing Neotrogla sex. Credit: Yoshizawa et al, 2014.

Neotrogla sex can last for days, so it’s important for the duo to stabilise themselves. The female does it by inflating the base of her gynosome inside the male. It’s covered in patches of tiny spines, which help to anchor her in place for her sexual marathon. You can find similar spines on the penises of many male animals where they provide extra stimulation during sex (as in cats, mice and chimps) or inflict horrendous wounds on the females (as in the seed beetle).

In Neotrogla, the spines are such good anchors that it’s impossible to separate a mating pair without killing the male. As Yoshizawa writes, “Pulling apart coupled specimens (N. curvata; n = 1) led to separation of the male abdomen from the thorax without breaking the genital coupling.” In other words: We tried yanking one pair apart; it didn’t work and the male kinda broke.

“This is a fascinating piece of natural history,” says Michael Jennions from Australian National University. “Evolutionary biologists have long been fascinated with why male genitalia, including penises, are so variable among species. Far less attention is given to female genitalia, in part due to the technical difficulty of measuring a cavity rather than a protrusion.”

Neotrogla’s set-up is unlike any other in the animal kingdom. There are some mites and beetles in which females have a protruding organ that collects sperm, but none of these have special adaptations for anchoring themselves inside males. The female seahorse uses a protruding structure to place eggs inside the male’s pouch—it’s the male who gets pregnant and nurtures the eggs—but that’s an egg-laying device, not a penis. And, as I mentioned, the spotted hyena has an enlarged pseudopenis, but it’s still the male that does the penetrating.

What’s more, the Neotrogla penis is a completely new structure with a complex set of accompanying membranes, muscles and ducts. Its evolution wasn’t a simple case of elongating an existing body part like the clitoris of a hyena. It has no equivalent. “Evolution of such novelties is exceptionally rare, maybe
comparable with the origin of insect wings,” says Yoshizawa.

The male vagina, meanwhile, has evolved in tandem so that it has just the right shape to accommodate the female’s penis. It’s easy to understand why such structures are extremely rare—reversing the usual pattern of genitals is not easily done. So why did it happen in Neotrogla, and onlyNeotrogla?

Yoshizawa suspects that the answer lies in the insects’ environment and lifestyle. They live in extremely dry caves and their main food—bat droppings and bat carcasses—are hard to come by. Starvation is always just round the corner, but females can fend it off by mating. That’s because the males package their sperm into packets called spermatophores, which are also loaded with nutrients. During sex, females get a meal along with sperm.

This flips the usual male-female dynamic. Males typically benefit from mating as much as possible, so that they can father the most offspring. They compete for mates, while females, who bear the burden of actually raising offspring, are choosy about their partners. But in Neotrogla, it’s the females who crave multiple mates, so they can get their jaws on as many spermatophores as possible. They’re the ones who compete, and the males are the choosier sex.

Yoshizawa thinks that this reversed sexual conflict led to the evolution of reversed genitals. The female’s gynosome allows her to anchor herself to her mate, even if he is reluctant, so that she can grab as many spermatophores as possible. “It is very likely that the entire mating process is actively controlled by females to obtain more seminal gifts from males, whereas males are rather passive,” he says. This might also explain why Neotrogla sex lasts for so long.

“This is possibly correct, but we know there are other species where females benefit from similar gifts but do not have a ‘penis’, such as several species of crickets, flour beetles and bean weevils,” says Jennions. “Unfortunately, when you only have one instance of a trait, it becomes difficult to attribute its evolution to a specific force, such as sexual conflict. Until we find more examples of female penises, the jury will remain out on the explanation.”

Meanwhile, Yoshizawa is busy testing his hypothesis. He’s doing a lot more work on the behaviour of the mating insects, the characteristics of the spermatophores, the growth of the gynosome, and pre-adaptations in other related species that allows Neotrogla to evolve a female species.

“This study shows that most things that we use to define males and females are not written in stone,” says Goran Arnqvist from Uppsala University. “They are reversible and evolvable results of the relative costs and benefits of mating. In terms of traits that separate males and females, there is little left but the type of gametes [sperm or eggs] they have.”

At io9, Annalee Newitz has written an interesting piece criticising much of the coverage of this story, including this post, and specifically the use of the term “female penis”. I disagree with many of her points and stand by the use of the term.

But first, to clarify, I absolutely agree with Newitz that cheap dick jokes are doing the topic a disservice, which is why you won’t find any here. The tone is as deadpan as I can muster—the only sniggering is reserved for the part of the study where one mating pair gets pulled apart and the male is accidentally bisected.

As to the other parts of Newitz’s critique, she repeatedly says that “female penis” is an inaccurate term that is “anthropomorphizing” Neotrogla’s anatomy—one should call the organ a “gynosome” (which I also do). I don’t agree that gynosome is accurate, while penis is not. As Diane Kelly, who studies penises points out: “As a technical term, a penis is a reproductive structure that transfers gametes from one member of a mating pair to another.” Which is exactly what is happening here.

Newitz points to differences. “When was the last time you found a penis that grew spines, absorbed nutrients, remained erect for 75 hours, or allowed its owner to get pregnant?” Actually spines are pretty common; long sexual bouts are pretty common; and the gynosome doesn’t absorb nutrients—it collects sperm packets that contain nutrients, which the animal then eats in the normal way. The key difference is that rather than delivering sperm, it collects it—as I stated right up top. And the only reason we think of penises as sending sex cells in that direction is that we never knew any other set-up could occur. Now we do, which either forces us to introduce a new term and demand that it be used, or to expand the bounds of our old term. I prefer the latter. I’m generally a lumper, rather than a splitter.

The gynosome is very much like a penis in both form and function. The authors highlight the differences by giving it its own specific name. But they also acknowledge its similarities to what we typically think of as penises by describing the organ as such, both in the title of their paper—“Female Penis, Male Vagina, and Their Correlated Evolution in a Cave Insect”—and throughout its text. They don’t get any special privilege because of their authorship, of course—but I’m pointing out that you can either look at this discovery through the lens of difference or similarity. And similarities are actually critical here because evolution crafts organs that are convergently similar—though different in the details—thanks to similar selection pressures.

In fact, there is a long tradition in anatomy of describing organs with almost metaphorical names. A snail’s foot is not remotely the same as a human’s foot, but they’re both muscular locomotive organs that are kinda on the bottom of the body. We call them both feet. An octopus radula is not a human tongue, but they’re both mobile things inside the mouth that perform feeding functions, so we call them both tongues. “Eye” gets used to refer to all manner of light-detecting organs regardless of huge differences in their anatomy, evolutionary history, physiology, because they all share the common theme of detecting light. And in a similar vein, a Neotrogla penis/gynosome is not the same as a human penis but they’re both used during penetrative sex for the transfer of gametes. Other penetrating sexual organs, like the aedagus (insect) and gonopodium (fish) are also colloquially known as penises.

So, do we make a special case for sex-related terms? Newitz would say yes, because of the cultural and social baggage that “female penis” carries, in a way that “snail foot” does not. This is the strongest part of the argument, and the part that gives me pause.

But Newitz also argues that the term “erases one of the most beautiful things about life, which is its awe-inspiring diversity”, and there I disagree. The post above specifically references that diversity—not just in Neotrogla but other animals like hyenas and seahorses, and goes into detail about sexual selection. It ends deliberately with a quote about how the split between males and females comes down to sex cells, and everything else is labile. If that’s not celebrating the diversity of life, I don’t know what is. I don’t think that referring to Neotrogla’s female sex organ as a penis whitewashes that diversity. If anything, it forces us to realise that one of the traits we often link to a penis–that it lives on a male–isn’t a necessary truth. The usage expands what we know, rather than erases.

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24 thoughts on “In This Insect, Females Have Penises And Males Have Vaginas”

So I can imagine the inevitable question: why are these considered females with penises? Why aren’t they considered males who lay eggs?

I imagine part of the answer may relate to their genome and organization of sex chromosomes in the family. But I suspect the other part of the answer lies in the actual differentiation of sperm and eggs. That is, as I understand it, the male is the sex that produces sperm and the female the sex that produces eggs. This is true then regardless of how the egg is fertilized or gestated (i.e. as noted, seahorses do it differently, and the male receives the fertilized egg but that egg was still produced by the female).

And the differentiation then of sperm and eggs? Sperm are very small, very mobile and produce in large quantities relative to eggs, which are very large and immobile. As I understand it, the specialization of gametes is probably a result of the need to pass on pristine new mitochondria to the next generation. That need explains why sperm are mobile and eggs are immobile. Eggs have the pristine mitochondria that do not produce energy, and hence cannot move.

Presumably this all holds true in Neotrogla curvata: that the males with vaginas produce sperm that has active mitochondria, while the females with penises produce eggs with inactivated pristine mitochondria that will become active once the egg is fertilized.

As you say, the operational definition of male is one that produces the smaller gamete (sperm). The form of genitalia doesn’t define male/female. So you can have females with penises but not males who make eggs.

Interesting disagreement between you and Annalee Newitz. I thought she did raise some good points but you’re right that convention has been to use the same name for very different appendages.

What I did find curious here was where you mentioned “reversing the usual pattern of genitals.” Is it really that usual? You’ve written a lot about sex and I remember you saying that only something like 4% of bird species have a penis and I believe fishes also seldom have penises. If there big groups that lack a penis isn’t maybe more a familiar pattern of genitals?

[I more meant that in species with a penis, it’s usually on the male, while the female has the opposing cavity. But you’re right: in some species, it’s about mushing together a pair of openings, and others practice external fertilisation – Ed]

In the 9th paragraph, shouldn’t that read “but none of these have special adaptations for anchoring themselves inside MALES”? But very interesting article. There is so much we do not know yet about so many things.
[Crap. Yes. Fixed – Ed]

I’ve been following the discussion about the use of the word “penis,” and I can see some of both sides of the issue.

But I was also wondering, would it be accurate as well to call it a highly modified vagina? The gynosome does still receive sperm, like a vagina, and cause the male to release nutrient packets. The females produce eggs through it, like a vagina. The only difference is in the penetrative aspect. Is it the penetration or the reception that make a penis or vagina? I feel like calling it a vagina might also highlight the vast diversity that exists in female reproductive organs.

[This is an awesome point. Okay, I’ve mulled it over. I think the reason that I’m looking at penetration as the defining trait is that we only really talk about penises and vaginas at all in species with internal fertilisation. If there’s external fertilisation, the terms don’t apply. If there’s just a mushing together of genital openings, like in most birds, the terms don’t really get used either – both sexes have cloacae. So while all sex involves the transfer and reception of gametes, the terms penis and vagina are used in the context of penetration, which is why that seems like the more salient factor here. But really good point, SciC. That’s my answer, but I’m not entirely sold on it. – Ed]

This isn’t a matter of calling a pterosaur a dinosaur; I’d say that in this case the female’s…sex organ…is very much a penis in the same way a bird’s kneecap is a patella. It evolved separately from mammal patellas, but they’re functionally the same thing. You could also call the armor covering of Henodus a “shell” even though Henodus is not a turtle and the shell is not homologous. Functionally, it’s a shell.

I like Sci’s point, and maybe this is what Annalee Newitz was getting to? That while it makes sense to use the term penis to refer to transfer of gametes, and that that doesn’t have to be sexed, we are talking about an organ that does both, yes? So maybe it makes sense to eventually settle on the term penis to describe this organ, but it’s interesting to think about (in terms of how we culturally construct power, and gender, and sex) why we wouldn’t do so for the term vagina.

wikipedia: “A penis is the primary sexual organ that animals use to inseminate sexually receptive mates”.

Thus, the gynosome does not qualify as a penis, because it is a sperm-collecting organ and not a sperm-releasing organ.
Some Plathelminthes can protrude their pharynx, it is still a pharynx and is not suddenly called “tongue” because it is something protruding from the mouth.

Your “penis” just got a whole bunch of non-scientists to read an article about science! I think we should celebrate that fact. I also think that the if scientists were to find a male insect with a vagina-like structure, there would be a whole slew of articles with vagina in the title. People are fascinated by sex, it sells, and yes, if used properly it can be used to “sell” science.

Fascinating! Both the discovery of this unusual anatomy, and your debate with Annalee on what to call it. I can see valid points on both sides of the argument, but have to agree with your response Ed. I find the diversity implications more fascinating than what we choose to call the organs. All names we give are in some way “anthropomorphizing” the natural world – so it is interesting to think about why/how we use these specific terms here from an anthropological perspective (as Kate Clancy points out). Gynosome sounds more neutral and technically correct, but penis is a more recognizable term likely to get more people to pay attention to this discovery. Dick jokes may be an unfortunate outcome, but those who snigger at those rather than marvel at this discovery, will find some way to snigger at anything vaguely sexual anyway. As Lila Higgins points out, this story has garnered so much attention largely because it has the words penis and vagina in the title – I don’t think gynosome would have quite done that trick. And Lila seems to have missed that this study actually also did “find a male insect with a vagina-like structure” – which raises the question: why the argument only over what we call the penis-like structure but not over the vagina-counterpart in the males?

I’m trained as a molecular biologist but I recall being corrected about insect penises, being told the correct term is “intromittant organ.” After this penis debate I started looking to see what the difference is and I can’t figure it out. Help? When is an intromittant organ a penis and when is it not?

“So while all sex involves the transfer and reception of gametes, the terms penis and vagina are used in the context of penetration, which is why that seems like the more salient factor here.”

Which makes me wonder, why, of all things, is it the “penetration” rather than the actual physiological role in the act of conception that is the most relevant trait here? Isn’t that exactly the critique that the io9 article had in the first place, that our ideas about penetration, domination and conquest are being projected?

[Because the act of penetration, despite the social constructs that surround it, also has actual important biological significance. The sexual conflicts between males and females play out differently if both shoot eggs/sperm into their surroundings and fertilise externally than if that act goes on internally. It’s why the evolution of genital shape and form is so rapid in groups like flies, and why it’s so fascinating to biologists who study sexual selection. Penetration isn’t just some cheap aspect of sex that people focus on because they’re projecting some social construct. In evolutionary terms, the act of penetration has important implications for sex beyond the act of conception. – Ed]

Reno: Yes, “intromittant organ” is the most general term for the sperm transfer thingie — but that’s because insect equipment — involving both sperm transfer and clasping accessories — is extraordinarily diverse, and homologies are frequently unknown. My first exposure was dissecting cockroaches which have what amounts to a Swiss Army assortment of paired and unpaired genital appendages.. But only a pedant would mind using the word “penis” in general discussion, as long as homology was not implied.. Pedants, of course, are more frequent in academia than penises in female insects.

Seems to me the definition of the male tab A and female slot B should remain inviolate which means the male has developed/evolved a womb and the female sperm, along with its nutritive pack, in order to rerproduce.

Another note – other insects may well be doing something similar to these bark lice. Snowfleas [Boreidae] are an entirely unrelated group, but share the features of female-on-top mating position, prolonged mating, sperm transfer via spermatophore. Most boreids have what appears to be rapine sex initiated by the male, but in at least one case, the reproductive structures interlock in what the author below calls “reciprocal intromission”. If a female boreid recived nutrients from the spermatophore, it wouldn’t take much to imagine their evolving the more female-controlled pattern that we see in the barklice.

I wonder whether endocrine blockers have somehow affected insects (as they have humans and amphibians) in ways we are now discovering? Have there been many such male-female physiological reverses in the past that we know if?? If none, there may be something to something akin to my suggestion.
[This is different to those sex reversals. When animals change sex due to endocrine disruptors, or naturally (see anthias), the female literally becomes a male and starts producing sperm, or the male literally becomes a female and starts making eggs. Here, the female is still a female and the male is still a male, but their sex organs are doing something different. – Ed]

I agree with you Ed.
But one thing you write is something of a contradiction.
You say:
“The female seahorse uses a protruding structure to place eggs inside the male’s pouch—it’s the male who gets pregnant and nurtures the eggs—but that’s an egg-laying device, not a penis.”
and:
“As Diane Kelly, who studies penises points out: “As a technical term, a penis is a reproductive structure that transfers gametes from one member of a mating pair to another.” Which is exactly what is happening here.”

If a penis is a reproductive structure that that transfers gametes from one member of a mating pair to another, then surely the seahorse ovipositor is doing just that too.

And, for that matter, spiders using the pedipalp to transfer sperm makes the pedipalp a penis.
And the hectocotylus of the octopus and squid, though they can also have a more ‘regular’ penis too.

“Intromittent organ” is probably the better term to use for all these structures but as I don’t think that will ever be popular I think using “penis” is our second-best option. Rather than this perpetuating all the baggage we carry in relation to the human penis, my hope is that the more we learn about all these different ‘penises’, ie gamete-transfer structures, the more we can grow up when it comes to thinking and talking about the human penis.

[The ovipositor is for transferring eggs that have already fertilised, and gametes are specifically sex cells that fuse together during fertilisation. And I totally agree with the last sentiment. – Ed]

Ed
In the seahorse the female transfers her eggs and they are then fertilised in the male’s pouch. The eggs have not already been fertilised before they are transfered. So I stick with my definition that the ovipositor in the seahorse is a penis 🙂 – it transfers unfertilised eggs into the male’s pouch.
The male ejaculates into the pouch where the eggs have been deposited.

“We tried yanking one pair apart; it didn’t work and the male kinda broke.”

This is why I love your writing.

Also, I don’t really see the problem with using the word “penis”. As a non-scientist but educated layperson, “penis” to me means “penetrating organ”. Don’t the two words share an etymological root? Even in hyenas where the female has a “pseudopenis”, it’s a PSEUDOpenis because it’s still the male’s penis that is doing the penetrating.
Just my undoubtedly short-sighted take on the matter.

Fascinating story!
One addition. These animals are not the only ones with a female penis. It also occurs in the hermaphroditic flatworm Bdellocephala punctata. These male-and-female animals have no male penis to deliver sperm to each other. Instead, they have a female penis to suck it out of each other. The reason probably is a strong preference to mate in the female rather than the male role.
See Anthes et al. 2006: http://www.sciencedirect.com/science/article/pii/S0003347206001345

I wouldn’t refer to this as a ‘penis’ either. It gives the impression that ‘data’ is being sent/delivered. This is the opposite, where ‘data’ is being collected.

The key here is the flow of ‘data’. If there is a female species that delivers eggs into the male rather than collects, then I’d be more inclined to refer to that as a ‘penis’.

I do find the most interesting point of the article being the defining characteristics of males/females, which is essentially about who produces the sperm and who produces the eggs. The delivery system itself doesn’t matter, because that can be varied. In that sense, though, the differences between male/female become almost non-existent at their cores: they’re just data that’s packaged slightly differently.

It also makes me wonder about potential other ‘sexes’ out there in the universe (if life does exist out there) and how they are constructed at their ‘cores’, if something like ‘sexes’ even exists outside our little home.

About Ed Yong

Ed Yong is a staff science writer at The Atlantic. His work has appeared in Wired, the New York Times, Nature, the BBC, New Scientist, Scientific American, the Guardian, the Times, and more. His first book I CONTAIN MULTITUDES—about how microbes influence the lives of every animal, from humans to squid to wasps—will be published in 2016 by Ecco (HarperCollins; USA) and Bodley Head (Random House; UK).

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